Phonon-assisted stimulated emission in Mn-doped ZnO nanowires

Well-crystallized Mn-doped ZnO nanowires were synthesized by a high-temperature chemical reduction method. The microscopic and macroscopic photoluminescence (PL) spectra of these Mn-doped ZnO nanowires were studied. Both the free-exciton emission band and the second-order longitude optical (2LO) phonon-assisted emission band were observed in the microscopic PL spectra. Furthermore, from the power dependent macroscopic PL spectra, the stimulated emission was observed around the 2LO phonon-assisted emission band with an increase in pump fluence, which relates to the strong exciton-phonon couplings along the c-axis in these Mn-doped ZnO nanowires. The nonresonant micro-Raman scattering spectrum demonstrates the strong 2LO phonon mode, which is enhanced by Mn doping. Resonant micro-Raman scattering shows multi-LO modes, due to the strong exciton-phonon coupling via a deformation potential induced by the Mn ions in these Mn-doped ZnO nanowires. Moreover, the low-temperature UV PL band is discussed to demonstrate the activity of phonons in the Mn-doped ZnO nanowires.